A persistent multi-version map: explicit map creation; change map factory to become just a factory for data types again

Also remove an unnecesssary copy of the closeSilently() method

Serialization is now pluggable using the system property "h2.javaObjectSerializer". Thanks to Sergi Vladykin for the patch!

A persistent multi-version map: open the file in exclusive or shared mode; allow to read and update the file header.

Fix ConcurrentModificationException when creating tables and executing SHOW TABLES in parallel. Reported by vvoytovych@llnw.com.

A persistent multi-version map: split based on size (memory), not number of entries; allow to estimate the memory used by unsaved changes